The invention relates to a procedure and a device for injection molding.
Probably injection molding is the most important procedure for manufacture of preforms. Roughly 60% of all plastic processing machines are injection molding machines, with 30% being extruders and 10% “exotics”. Preforms weighing only a few milligrams up to 30 kg of shot are produced on injection molding devices.
Injection molding is above all suited to mass-produced products, since raw material (granulate) can be converted for the most part in one working pass into a completed part. Subsequent reworking is minor or can be dispensed with altogether and even complex geometries can be produced in a single working pass. In addition, many types of filler materials such as glass fibers, talcum, soot, metal shavings, pigments, polymeric additives, etc. can be included, thus making it possible to make specific modifications to the properties of the finished product.
The properties of a finished product are determined by the material used, the shaping and the type and implementation of the processing. With plastics, especially thermoplastics, these influences can be recognized in more pronounced fashion than with metals. The selection of the “right” plastic—partially crystalline or amorphous—as the material depends to a high degree on its molecular structure. In an injection molding process, almost exclusively thermoplastic polymers are processed. Thermoplastics consist of linear macromolecules, which are present in the completed part either in statically convoluted (amorphous), regularly arranged (crystalline), or stretched (oriented) form. In most cases, all three states are united in a completed part. Due to the relatively high molecular weight of all plastics, a 100% crystalline state is never achieved in the finished piece. In this connection, one speaks of a degree of crystallinity, which is a ratio of crystalline volume to overall volume. Usually the degree of crystallinity of preforms is between 50% and 80%, and along with material-specific properties, this depends primarily on the design (tool) and the processing parameters.
One additional important viewpoint for correct selection of the material is the subsequent temperature at which the completed part is to be used. Here especially, attention is to be paid to the glass transition temperature. Since some types of plastics have glass transition temperatures that are in the room-temperature range, the question of usage above or below the glass transition temperature can be very decisive, since in the area of the glass transition temperature, many mechanical properties can change “suddenly”.
The most important processing parameters in injection molding are the mass temperature, the tool temperature, the form filling time and the injection volume and the pressure gradient in the tool (interior form pressure).
The form filling time and the pressure gradient in the tool are decisive in determining the process of form filling, and thus the mechanical properties of the finished part. Since most plastics shrink during the cooling process, to reach the sealing point (congealing of the shrinking head), shaped masses must be fed under dwell pressure. After the sealing point is reached, the dwell pressure is shut off. However, the preform still is in a dwell state for a certain time (residual cooling time) in the closed tool. During this residual cooling time, the preform temperature drops below the melt temperature and the glass transition temperature, so that when the completed part is ejected, deformation is prevented. The entire period of time from the start of tool filling until the next commencement is called the cycle time.
The mass and tool temperatures to be selected are tool-specific parameters, and therefore are pre-set or recommended in most cases by the plastics manufacturer. Due to them, the properties of the completed part can be influenced. Thus, for example, with PET, the tool temperature is very decisive for the crystal structure of the finished part: a low tool temperature causes quick cooling, and the finished part is amorphous and transparent. High tool temperature increases the degree of crystallinity and thus, for example, the mechanical properties of the preform. The usual mass temperatures in processing mass plastics like PP, PE, PS, . . . are in the range from 220° to 280° C., with tool temperatures between 30° C. and 120° C. High-performance plastics (PEEK, PPS, LPSs, . . . ) require mass temperatures of up to 480° C. and tool temperatures of up to 200° C. Thermoplastic injection molding is the basis for all other injection molding procedures, and presently overall is the most frequently used plastic processing procedure.
Injection molding machines—including injection molding machines used in the present instance—generally consist of two pieces: the spraying and plastifier unit that prepares the plastic granulate and sprays it under pressure into the tool, and the enclosing unit, which receives the tool (also the form) and opens and closes it.
The core piece in the spraying unit is a worm-gear shaft, also called a worm, that extends in a cylinder or housing. The inner diameter of the cylinder is equal to the outer diameter of the worm. The cylinder is most commonly designated as the worm cylinder. In the rear area of the worm cylinder is a hopper into which the plastic granulate is filled. Through an opening (the filler block) the granulate trickles into the cylinder. Turned by a drive, the worm rotates in the worm cylinder and transports the granulate forwards. In thermoplastic injection molding, the worm cylinder is heated from without by means of electrical heater bands. Due to this heat and the special geometry of the worm, the granulate is not merely moved but also clipped; the plastic melts, plastifies and becomes homogenized. At the tip of the worm cylinder is a nozzle that forms the transition to the tool.
During the dosing process, the shape mass is mostly transported through a non-return valve to the nozzle, and accumulates in front of it. To offer sufficient retaining capacity for the shape mass, the work is impinged on only axially by a slight amount of pressure (banking-up pressure), so that it can shift in the direction of the filling hopper and thus forms the so-called worm outer chamber in which the mass volume is found. The banking-up pressure acts against the melt, so that the melt is compressed and does not pull the worm back. The pressure which the melt exerts moves the worm back.
With the injection process, the worm is pressed axially toward the nozzle, whereby the non-return valve is closed and thus the mass volume is sprayed through the nozzle into the tool.
The non-return valve is a component of the injection molder. Essentially it consists of a locking rink, a worm tip and a compression ring, and it sits at the tip of the dosing worm. The quality of the injection-molded part is decisively dependent on its function. During the injection process, the non-return valve prevents the molten material from flowing back into the worm passages. When dosing, it likewise makes it possible for material to flow from the worm area into the filler space. If the dosing worm is turned, then it feeds the plastic material through the opened non-return valve into the filler space and the worm moves backward in an axial direction until it reaches the set value. During injection, the worm is shifted forward by a hydraulic unit. Now the build-up and the locking ring close the path in the worm direction. The dosed material is now pressed into the injection molding form with no loss of pressure or amount.
After a part filling of 90% to 98%, a switchover is made to restoring compression. A mass must remain in the cylinder (residual mass filler), because otherwise the pressure cannot act on the mass. The restoring compression is necessary to compensate for the shrinkage in volume.
A three-zone worm is often used in thermoplastic processing. In the so-called intake zone, the plastic granulate is taken in and fed into the next zone, the compression zone, where the plastic is plastified and compressed (and degassed if necessary). After that, the melt is homogenized in the metering zone and finally compressed through the non-return valve in front of the worm, which moves axially backward as a result of increasing banking-up pressure.
Various procedures and devices for injection molding are known from prior art.
For example, from DE 198 03 422 A1 a procedure and a device are known for transfer molding of fiber-reinforced plastics.
The disclosed procedure makes provision for a plastic raw material such as comminuted or granulated plastic raw material to be passed to a plastifier device consisting of an extruder housing and an extruder worm that turns in the extruder housing about a longitudinal axis, and to plastify it in the plastifier device and feed it in the direction of an extrusion die. In the area of the plastifier device, fiber material is fed to the plastified plastic mass and mixed during further transport in the plastifier device with the plastified plastic mass. After this, the plastified mass containing fibers is fed to an injection device and injected by the injection device through an injection nozzle into an extrusion die formed from at least two pieces and then compressed into a formed body in the extrusion die. The plastic raw material in this case is fed to the extruder via a shredder, with additives being added if necessary to the raw material in the shredder.
The disclosed device for carrying out the procedure has a plastifying device with an extruder housing and an extruder worm placed so as to turn about a longitudinal axis in the extruder housing, a dosing device for fiber material, and an injection device with an injection cylinder, an injection piston and an injection nozzle as well as a compression mold tool with halves of the extruder die placed movably one on the other. Additionally, at the start of the extruder cylinder is a shredder, which comminutes the plastic raw material, heats it by tribological heat and feeds it to the extruder worm in the extruder cylinder.
This procedure and the device have thoroughly proven themselves, but they are not suitable for processing of plastic mixtures, especially mixtures with at least one polyester component, because polyesters in particular, close to their melting point, react in sensitive fashion to natural atmospheric moisture; i.e. the chain length of the molecules is shortened through hydrolytic breakdown, which results in disadvantageous changes in the material properties, such as reduced strength or altered color. Such a disadvantageous effect on material properties is not desired in the end product to be manufactured.
Other types of plastics such as polyamides are in danger of oxidation at or near their melting point, which also entails the above-mentioned disadvantages in regard to the properties of the material or end product.
To avoid these disadvantages, a device is known from EP 390 873 for preparation of thermoplastic material. This device comprises a receiving container that at the top can be closed by a sluice to be at least essentially gas-tight for plastic material to be brought in. For evacuation or for introducing protective gas into this interior space, the interior space of the receiving container is connected by means of at least one conduit at a location that is higher than the highest filling level in the receiving container, with this conduit being attached to a suction pump for a gas-forming medium or to a pump for a protective gas, and to the sluice an additional conduit leading to the pump is attached. Such a device makes it possible to ensure especially drying and heating without decomposition reactions of the polyester. The receiving container is appropriately provided with a shredder knife, an agitator blade, or a stirring beam, which can also be sealed gas-tight to the inner space of the container. In addition, the feed opening of the shredder is also designed to be gas-tight to the extruder cylinder. In practice, this device has also proven itself.
A further possibility to prepare plastic mixtures is described in WO 01/68345, namely a procedure for transfer molding of plastic mixtures, especially plastic mixtures having at least one polyester and at least one modifier component, especially recyclates of same, as well as a device for carrying out this procedure which permits processing of such plastic mixtures while largely maintaining the material properties, and which makes known improvements in material properties of such plastic mixtures usable to the fullest extent. Especially if the melting points of one or more of the modifier components are close to the drying temperature for the polyester component in the receiving container, by this means, possible backups or agglomerations in the preparation device are prevented.
The thermoplastic polyester component or the PET mixture in the initial state is heated and dried in a pretreatment station, analogous to EP 390 873. Then the heated and dried PET component is released to a plastification unit and at least one modifier component is added in. The mixture of thermoplastic polyester component and modifier component is homogenized in the plastification unit and is brought out as a melt into an injection unit and then injected into an opened extruder die.
Processing of a plastic material in an injection molding unit essentially depends on the various parameters or properties of the material fed to the injection molding unit, especially on its viscosity, crystallinity, molecular breakdown, orientation in the surface layer, on possible anisotropies, etc. All these parameters are decisively influenced by the type of processing or preparation of the materials before the plastification or before the melting. But also the kind of melting and the kind of injection process affect the end-result quality of the end product. In this regard, practical and commercial aspects such as cycle times, etc. are to be taken into account.
Especially when we are dealing not with end-product items manufactured in this way, but rather, for their part, with intermediate products that, for example, still have changes in shape to undergo, such as preforms, it is advantageous for these products to be of high quality.